Method for diagnosing exhaust gas probes and/or catalytic converters

ABSTRACT

New operating phases of internal combustion engines, such as coasting in which the internal combustion engine is decoupled from the rest of the drive section, continuously decrease the number of phases in which there is an overrun mode of an internal combustion engine in which diagnoses of catalytic converters and exhaust gas probes are carried out. Also, a method wherein the diagnosis of the at least one exhaust gas probe and/or the at least one catalytic converter carried out during the coasting mode and, in particular, when the cylinder bank is switched off.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit under 35 U.S.C.§119 of German Application No. DE 10 2011 001 045.9, filed Mar. 3, 2011, which is incorporated by reference herein in its entirety.

BACKGROUND

The invention relates to a method for diagnosing exhaust gas probes and/or catalytic converters having at least one catalytic converter which is arranged in an exhaust gas duct of an internal combustion engine and an exhaust gas probe which is arranged in the exhaust gas duct, the function of which exhaust gas probe is checked.

In order to meet legal requirements, it is necessary within the course of on-board diagnostics to subject the exhaust gas probes which are arranged in the exhaust gas section of an internal combustion engine to various tests with the engine running. These tests are usually carried out in the overrun mode of the engine, that is to say outside the load state while no fuel is being injected so that lambda is infinitely large and a jump in the lambda value which can be evaluated takes place when the overrun mode ends.

Furthermore, tests are known with which a correct connection of the lambda probes is checked after their installation. For example DE 44 23 344 A1 proposes interrupting the injection of fuel into a cylinder bank in an internal combustion engine with two separate cylinder banks and exhaust gas sections in order to determine whether the respective lambda probe exceeds or undershoots a defined threshold value, wherein when said value is exceeded or undershot it is determined that the two probes have been interchanged. Recurring diagnosis during the travel mode is not disclosed.

DE 198 01 629 A1 discloses a method for diagnosing a catalytic converter during the on-board diagnosis with a lambda probe which is arranged downstream of the catalytic converter, in which method a misfire of the combustion is simulated by stopping the supply of fuel into a cylinder. This requires a certain state of the volume of the catalytic converter. The driving behavior is, however, influenced by such a control. The emission values also increase since the lambda control of the engine is temporarily switched off.

The number of these overrun phases during which the on-board diagnostics usually take place has, however, significantly decreased, in particular in vehicles with double-clutch transmissions or torque-converter transmissions, since in these vehicles switching over to coasting mode occurs in order to achieve a further saving in fuel. In this context, the engine is decoupled and correspondingly runs in the idling mode instead of in the overrun mode, with the result that the engine braking effect does not occur. Due to the reduced number of overrun phases, it may be found that the diagnoses cannot all be completely carried out within a time period which is prescribed for their execution. If the legally required minimum in terms of the frequency of the diagnoses is undershot, this vehicle would not be approved for registration.

For this reason, means of blocking the coasting mode were introduced, which are not suspended until the necessary diagnoses are completed. This leads to increased fuel consumption and, under certain circumstances, to customer dissatisfaction since the customer cannot ascertain at what point the coasting function will start so that he is subjected to a different driving behavior of the vehicle. Furthermore there is the possibility of a forced overrun phase, which, however, also increases the emission values and restricts the driving performance.

SUMMARY

The problem therefore arises of making available a method for on-board diagnosis of exhaust gas probes or catalytic converters by means of which a sufficient number of diagnoses can be carried out without the coasting mode which saves fuel having to be eliminated. Disadvantages with respect to the driving behavior or emissions which occur are also to be avoided.

This problem is solved by means of the characterizing part of a method for diagnosing exhaust gas probes and/or catalytic converters having at least one catalytic converter which is arranged in an exhaust gas duct of an internal combustion engine and an exhaust gas probe which is arranged in the exhaust gas duct, the function of which exhaust gas probe is checked, characterized in that the diagnosis of the at least one exhaust gas probe and/or of the at least one catalytic converter is carried out during the coasting mode.

Since the diagnosis of the at least one exhaust gas probe and/or of the at least one catalytic converter is carried out during the coasting mode, that is to say when the engine is decoupled, the operating states which are to be run through during the diagnosis cannot be felt by the driver. It is also not necessary to dispense with coasting phases, with the result that no consumption disadvantages occur.

In one particularly preferred method, in a first diagnostic section the cylinders which are assigned to a first exhaust gas section are switched off. In this way, an overrun mode in which the diagnosis of the exhaust gas probes and of the catalytic converter usually takes place can be simulated for this exhaust gas section since, as in the overrun mode, only air is pumped through the engine. It is therefore possible to dispense with the previous adjustment to a lean setting for the diagnosis of the catalytic converter, which significantly reduces the risk of the formation of nitrogen oxides during the diagnostic phase as well as the risk of misfires.

In a first phase in the first diagnostic section the lambda value of a first exhaust gas section is preferably set to a predetermined value, in a second phase for generating a sudden change in the lambda value the cylinders which are assigned to the first exhaust gas section are firstly switched off, in a third phase this state is maintained over a certain time period, and subsequently in a fourth phase the cylinders which are assigned to the first exhaust gas section are switched on again with a defined lambda value. As a result of this control, all the relevant diagnoses can be carried out during the coasting mode. It is possible to check both the storage activity of the catalytic converter and the correct functioning of exhaust gas probes which are arranged upstream or downstream of the catalytic converter in the first exhaust gas section.

In a further method, in a second diagnostic section the cylinders which are assigned to a second exhaust gas section are switched off. It is therefore possible for the diagnoses for two separate cylinder banks or exhaust gas sections to be carried out in succession, with the idling mode on one of the cylinder banks being maintained in each case.

In a further advantageous method, in a first phase in the second diagnostic section the lambda value of a second exhaust gas section is set to a predetermined value, in a second phase for generating a sudden change in the lambda value the cylinders which are assigned to the second exhaust gas section are firstly switched off, in a third phase this state is maintained over a certain time period, and subsequently in a fourth phase the cylinders which are assigned to the second exhaust gas section are switched on again with a defined lambda value. In this way it is also possible to carry out all the relevant diagnoses during the coasting, mode for the second exhaust gas section.

The diagnosis is preferably interrupted when the coasting mode ends, and the cylinders which are assigned to the first or second exhaust gas section are switched on again, and when the coasting mode is re-entered that diagnostic section which was interrupted when the coasting mode ended is started. This control ensures that a sufficient number of diagnoses are also carried out in the driving cycle for the second exhaust gas section.

At an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the second phase to detect the sudden change in the lambda value at the changeover from the rich mixture to the lean mixture is preferably measured. In this way it is possible to ascertain whether there is a sufficient reaction speed of the exhaust gas probe.

Furthermore, according to one aspect of the invention at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section it is measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value. The voltage of an exhaust gas probe with is embodied as a lambda probe should be in the region around 0 at this time, but should exceed a defined threshold in order to ensure that a rising voltage is tapped when the excess air factor drops.

At an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the fourth phase to detect the sudden change in the lambda value at the changeover from the lean mixture to the rich mixture can preferably be measured. In this way it is also possible to check a sufficient reaction speed at the changeover from a lean mixture to a rich mixture.

At an exhaust gas probe which is arranged upstream of a catalytic converter in the first or second exhaust gas section it is advantageously measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value, as a result of which it is also ensured for this second probe that defined voltage values are output by the probe when the excess air factor drops.

In the first phase a lambda value of 1 should also be set at the exhaust gas probe which is arranged downstream of the catalytic converter. This ensures a sufficiently measurable time period for measuring the reaction time of the exhaust gas sensor when a cylinder bank is switched off.

A method for diagnosing exhaust gas probes and/or catalytic converters is therefore made available with which, even during rarely occurring overrun phases, diagnoses can be carried out sufficiently frequently in accordance with the legal requirements without emission values increasing or adverse effects becoming perceptible to the driver of a vehicle.

A method according to the invention will be described below with reference to the figure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of the profile of a method according to the invention and of the resulting graph of the voltage at the lambda probes downstream of the catalytic converters of two exhaust gas sections.

DETAILED DESCRIPTION

At first, the internal combustion engine, which has two cylinder banks which are separate from one another and each have an exhaust gas section, is operated, for example, in the stoichiometric range with lambda=1. Each exhaust gas duct is assigned a catalytic converter, upstream of whose inlet and downstream of whose outlet there is respectively arranged an exhaust gas probe which is embodied as a lambda probe.

When the accelerator pedal is released, the vehicle firstly goes into the overrun mode, wherein in the case of vehicles which are equipped, for example, with a Porsche double-clutch transmission, the internal combustion engine is subsequently decoupled from the rest of the drive section in order to bring about freewheeling of the vehicle, with the result that the resistance of the engine which generates a braking effect on the vehicle is eliminated. The vehicle can travel relatively long distances outside the load range due to this reduced resistance, as a result of which the consumption is reduced. This operating state with decoupled engine during driving is referred to as a coasting phase.

After the entry into the coasting phase, in the present exemplary embodiment firstly an initial diagnostic section 2 begins, during which diagnostic section 2 in a first phase 4 a defined lambda value, which is, for example, 1, is modulated with the result that the decoupled engine is operated in the stoichiometric range.

In a second phase 6, the first cylinder bank, which is connected to the first exhaust gas section, is switched off. This means that there can be no further injection of fuel in these cylinders. This should lead, at the exhaust gas sensor which is arranged downstream of the exhaust gas catalytic converter, to a sudden drop in the voltage which is generated at the lambda probe since the engine is supplied exclusively with air, with the result that lambda becomes equal to an infinite value. For the purpose of diagnostics, the time period which the lambda probe requires to drop from the defined voltage value at lambda=1 to the defined voltage value lambda which corresponds to an infinite value lambda is measured. If this time period for a changeover from a stoichiometric air ratio to a lean air ratio does not exceed a prescribed threshold value, it is inferred that the sensor is operating correctly. Otherwise, a fault signal is output.

In the step which follows, in a third phase 8 in which the cylinder bank is further located in the switched-off state, and therefore the engine is operated in an infinitely lean fashion, it is checked whether the voltages of the two sensors upstream of the catalytic converter and downstream of the catalytic converter exceed a predefined threshold value in this operating state. If this is the case, the correct functionality of the lambda sensors is in turn inferred, and otherwise a fault signal is generated.

Subsequently, in a fourth phase 10 the first cylinder bank which is assigned to the first exhaust gas section is fired again, that is to say supplied with fuel, and set to a defined lambda value which is in turn 1 in the present exemplary embodiment. This re-ignition of the cylinder bank therefore results once more in a jump of the lambda value of the lambda sensor arranged downstream of the catalytic converter, specifically from infinite to 1, with the result that the voltage rises from the defined threshold value at an infinite lambda value to the voltage at a lambda value of 1. The voltage jump which takes place with a slight delay with respect to the re-ignition is also detected here, and the time period until the expected voltage value is reached at the jump from the lean state into the relatively rich, stoichiometric state is measured. If this time period is sufficiently short, the sensor is stored as being functionally capable, and is otherwise stored as being faulty.

If the internal combustion engine is still in the coasting mode, the driver therefore does not change over into the load mode by actuating the accelerator pedal, and subsequently in a second diagnostic section 12 the second cylinder bank is operated in a first phase 14 with lambda=1 and subsequently is no longer supplied with fuel and is placed in a quasi-overrun state, with the result that a second phase 16 corresponding to the second phase 6 of the first diagnostic section 2 is carried out for the second exhaust gas section. Subsequently, a third phase 18, corresponding to the third phase 8 of the first cylinder bank, and a fourth phase 20, corresponding to the fourth phase 10 of the first cylinder bank, are carried out at the second cylinder bank.

If the coasting mode and therefore also the running diagnostic section 2, 12 are interrupted during one of the two diagnostic sections 2, 12, the coasting mode which starts anew is continued with the diagnostic section 2, 12 which was interrupted.

Of course, it is also possible, by comparing the phase shifts between the respective exhaust gas sensor upstream of the catalytic converter and that downstream of the catalytic converter, to determine the conversion capability of the catalytic converter in the respective exhaust gas section, with the result that the catalytic converter can be classified as functioning or faulty by means of the shifting of the voltage jumps. This can be done simultaneously in a corresponding fashion with the diagnosis of the exhaust gas probes in the coasting mode and by switching off the respective cylinder bank.

It becomes clear that this method permits the diagnoses which are otherwise carried out in the overrun mode to be replaced or to be carried out in parallel therewith. This ensures that the number of diagnoses which are legally prescribed within one driving cycle are also actually carried out.

It should become clear that the scope of protection of the main claim is not limited to the described exemplary embodiment but that instead various other diagnostic steps can be performed. 

1. A method for diagnosing exhaust gas probes and/or catalytic converters having at least one catalytic converter which is arranged in an exhaust gas duct of an internal combustion engine and an exhaust gas probe which is arranged in the exhaust gas duct, the function of which exhaust gas probe is checked, wherein the diagnosis of the at least one exhaust gas probe and/or of the at least one catalytic converter is carried out during the coasting mode.
 2. The method for diagnosing exhaust gas probes as claimed in claim 1, wherein in a first diagnostic section the cylinders which are assigned to a first exhaust gas section are switched off.
 3. The method for diagnosing exhaust gas probes as claimed in claim 2, wherein in a first phase in the first diagnostic section the lambda value of a first exhaust gas section is set to a predetermined value, in a second phase for generating a sudden change in the lambda value the cylinders which are assigned to the first exhaust gas section are firstly switched off, in a third phase this state is maintained over a certain time period, and subsequently in a fourth phase the cylinders which are assigned to the first exhaust gas section are switched on again with a defined lambda value.
 4. The method for diagnosing exhaust gas probes as claimed in claim 2, wherein in a second diagnostic section the cylinders which are assigned to a second exhaust gas section are switched off.
 5. The method for diagnosing exhaust gas probes as claimed in claim 4, wherein in a first phase in the second diagnostic section the lambda value of a second exhaust gas section is set to a predetermined value, in a second phase for generating a sudden change in the lambda value the cylinders which are assigned to the second exhaust gas section are firstly switched off, in a third phase this state is maintained over a certain time period, and subsequently in a fourth phase the cylinders which are assigned to the second exhaust gas section are switched on again with a defined lambda value.
 6. The method for diagnosing exhaust gas probes as claimed in claim 4, wherein the diagnosis is interrupted when the coasting mode ends, and the cylinders which are assigned to the first or second exhaust gas section are switched on again, and when the coasting mode is re-entered that diagnostic section which was interrupted when the coasting mode ended is started.
 7. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the second phase (6, 16) to detect the sudden change in the lambda value at the changeover from the rich mixture to the lean mixture is measured.
 8. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section it is measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value.
 9. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the fourth phase to detect the sudden change in the lambda value at the changeover from the lean mixture to the rich mixture is measured.
 10. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein at an exhaust gas probe which is arranged upstream of a catalytic converter in the first or second exhaust gas section it is measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value.
 11. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein in the first phase a lambda value of 1 is set at the exhaust gas probe which is arranged downstream of the catalytic converter.
 12. The method for diagnosing exhaust gas probes as claimed in claim 3, wherein in a second diagnostic section the cylinders which are assigned to a second exhaust gas section are switched off.
 13. The method for diagnosing exhaust gas probes as claimed in claim 12, wherein in a first phase in the second diagnostic section the lambda value of a second exhaust gas section is set to a predetermined value, in a second phase for generating a sudden change in the lambda value the cylinders which are assigned to the second exhaust gas section are firstly switched off, in a third phase this state is maintained over a certain time period, and subsequently in a fourth phase the cylinders which are assigned to the second exhaust gas section are switched on again with a defined lambda value.
 14. The method for diagnosing exhaust gas probes as claimed in claim 13, wherein the diagnosis is interrupted when the coasting mode ends, and the cylinders which are assigned to the first or second exhaust gas section are switched on again, and when the coasting mode is re-entered that diagnostic section which was interrupted when the coasting mode ended is started.
 15. The method for diagnosing exhaust gas probes as claimed in claim 14, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the second phase to detect the sudden change in the lambda value at the changeover from the rich mixture to the lean mixture is measured.
 16. The method for diagnosing exhaust gas probes as claimed in claim 15, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section it is measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value.
 17. The method for diagnosing exhaust gas probes as claimed in claim 16, wherein at an exhaust gas probe which is arranged downstream of a catalytic converter in the first or second exhaust gas section, the time period which the exhaust gas probe requires in the fourth phase to detect the sudden change in the lambda value at the changeover from the lean mixture to the rich mixture is measured.
 18. The method for diagnosing exhaust gas probes as claimed in claim 17, wherein at an exhaust gas probe which is arranged upstream of a catalytic converter in the first or second exhaust gas section it is measured in the third phase whether the exhaust gas probe exceeds a predefined voltage threshold given an infinite lambda value.
 19. The method for diagnosing exhaust gas probes as claimed in claim 18, wherein in the first phase a lambda value of 1 is set at the exhaust gas probe which is arranged downstream of the catalytic converter. 